Method and apparatus for forming image

ABSTRACT

A movable light beam is controlled and set within a tolerable range of a beam position A by using a galvano-mirror, and then a circuit offset relating to the beam position A is measured. With the circuit offset being subtracted, the movable laser beam is exactly controlled and set at the beam position A, and an output value DA from a sensor at that time is stored. Subsequently, an output value DB from the sensor is similarly stored in connection with a beam position B. An ultimate beam position of the movable laser beam M is controlled such that a difference between the output values DA and DB coincides with a difference between an output value DS of the sensor with respect to a stationary laser beam S and an output value DM of the sensor in connection with the movable laser beam M.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-116212, filed Apr. 18, 2002, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a beam position adjusting method for an optical scanning device used in, e.g. an electrophotographic digital copying machine, an image forming apparatus having the optical scanning device, and an image forming method.

[0004] 2. Description of the Related Art

[0005] In the technical field of image forming apparatuses such as high-speed digital copying machines and high-speed printers, there is known an optical scanning device using, e.g. a multi-beam exposure device which simultaneously scans two or more beams and exposes a charged area on a photosensitive drum.

[0006] The multi-beam exposure device comprises, for example, a plurality of semiconductor laser elements for emitting laser beams; an optical member for adjusting the cross-sectional beam dimensions of the laser beams emitted from the semiconductor laser elements in accordance with a required resolution and setting an inter-beam distance between the laser beams in a sub-scan direction (a direction perpendicular to the axis of the photosensitive drum); a deflection device for deflecting and scanning the laser beams together in a main scan direction (an axial direction of the photosensitive drum); and a focusing lens system for guiding and focusing the laser beams deflected by the deflection device on a charged area on the photosensitive drum.

[0007] A galvano-mirror is used as an optical member used in the multi-beam exposure device. The galvano-mirror can minutely alter the angle of the mirror surface by controlling current supplied to an electromagnetic coil.

[0008] For example, in a multi-beam exposure device that simultaneously scans two laser beams, that is, a stationary laser beam and a movable laser beam, the angle of the mirror surface of the galvano-mirror is finely adjusted to control the distance (or the position) of the movable laser beam in the sub-scan direction.

[0009] The adjustment of the distance is effected by using a sensor such as a photodiode. Specifically, a laser beam is caused to scan the light-receiving surface of the sensor and a position on the light-receiving surface, at which the laser beam passes, is detected. This operation is performed for each laser beam. Thus, the distance between the laser beams is detected, and is adjusted based on the detection result.

[0010] When the inter-beam distance is to be adjusted, a circuit offset has to be considered. In other words, an output from the sensor is converted to a voltage signal by a current/voltage conversion circuit, and the voltage signal is amplified by an amplifier circuit and finally output via an integration circuit, etc. Therefore, a circuit offset in these circuits needs to be considered.

[0011] A description will now be given of a case where a distance (or a position) of a movable laser beam in the sub-scan direction is set relative to a stationary laser beam.

[0012] In the prior art, in the state in which a stationary laser beam is incident on a certain position on the light-receiving surface of the sensor, a galvano-mirror is used to effect a control to position a movable beam within a tolerable range of a beam position A that is a target position. Then, the movable laser beam is turned off, and a circuit offset relating to the beam position A is measured (S1).

[0013] Then, the galvano-mirror is used to effect a control to position the movable laser beam at a beam position B that is a target position. Then, the movable laser beam is turned off, and a circuit offset relating to the beam position B is measured (S2).

[0014] Subsequently, the galvano-mirror is used to effect a control to exactly position the movable laser beam once again at the beam position A by subtracting the circuit offset measured in step S1. An output value DA of the sensor at that time is read and stored in a memory.

[0015] Following the above, the galvano-mirror is used to effect a control to exactly position the movable laser beam once again at the beam position B by subtracting the circuit offset measured in step S2. An output value DB of the sensor at that time is read and stored in the memory.

[0016] The distance between the beam position A and beam position B is determined according to a resolution to be set. For example, if the resolution is 600 dpi, the distance is 42.3 μm.

[0017] The galvano-mirror is controlled to set the beam position of the movable laser beam such that a difference (DA-DB) between the obtained output values DA and DB of the sensor may become equal to a difference (DS-DM) between the output value DS obtained from the sensor when the stationary laser beam is incident on the sensor and the output value DM obtained from the sensor when the movable laser beam is incident on the sensor.

[0018] Thereby, the distance between the stationary laser beam and movable laser beam is set at a value corresponding to a predetermined resolution.

[0019] In the above-described prior-art beam position adjustment, however, the following controls are required:

[0020] (1) The control to position the movable laser beam within the tolerable range of the beam position A that is the target position, by using the galvano-mirror, thereby measuring the circuit offset at the beam position A.

[0021] (2) The control to position the movable laser beam within the tolerable range of the beam position B that is the target position, by using the galvano-mirror, thereby measuring the circuit offset at the beam position B.

[0022] (3) The control to position the movable laser beam at the beam position A with the circuit offset being taken into account, by controlling the galvano-mirror.

[0023] (4) The control to position the movable laser beam at the beam position B with the circuit offset being taken into account, by controlling the galvano-mirror.

[0024] The above controls (1) to (4) are effected in succession, thus adjusting the distance between the stationary laser beam and movable laser beam in accordance with the resolution. In other words, the position of the movable laser beam has to be varied from the beam position A, to the beam position B, to the beam position A, to the beam position B, and to the ultimate adjusted position, while operating the galvano-mirror which is very difficult to adjust. Thus, the adjustment of the laser beam position is time-consuming.

[0025] The positional adjustment of the laser beam is performed at the time of warm-up after power-on, completion of elimination of jamming, completion of pre-heating, etc., in the case of digital copying machines. However, since the adjustment of the laser beam position is time-consuming, a great deal of time is required until the start of the copying of a first sheet.

BRIEF SUMMARY OF THE INVENTION

[0026] The present invention has been made in consideration of the above problems, and its object is to provide a beam position adjustment method for an optical scanning device, which is able to perform laser beam position adjustment in a short time, an image forming apparatus, and an image forming method.

[0027] In order to achieve the object, the present invention may provide a beam position adjusting method for an optical scanning device using a galvano-mirror capable of altering an angle of a mirror surface by a drive control, and using a sensor that detects passage positions of a stationary light beam whose scanning position is fixed and a movable light beam whose scanning position is varied by adjusting the angle of the mirror surface of the galvano-mirror, the method comprising: effecting a control to position the movable light beam within the tolerable range of a first passage target position by adjusting the angle of the mirror surface of the galvano-mirror, and measuring a first circuit offset in a detection circuit for an output of the sensor at a time when emission of the movable light beam is stopped; emitting the movable light beam once again, effecting a control to position the movable light beam at the first passage target position with the first circuit offset being subtracted, and storing a first output value from the sensor at a time when the movable light beam is controlled and set at the first passage target position; effecting a control to position the movable light beam within the tolerable range of a second passage target position by adjusting the angle of the mirror surface of the galvano-mirror, and measuring a second circuit offset in the detection circuit for the output of the sensor at a time when emission of the movable light beam is stopped; emitting the movable light beam once again, effecting a control to position the movable light beam at the second passage target position with the second circuit offset being subtracted, and storing a second output value from the sensor at a time when the movable light beam is controlled and set at the second passage target position; and controlling the angle of the mirror surface of the galvano-mirror such that a difference between a third output value from the sensor at a time when the stationary light beam passes through the light-receiving surface of the sensor and a fourth output value from the sensor at a time when the movable light beam passes through the light-receiving surface of the sensor coincides with a difference between the first output value and the second output value.

[0028] Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0029] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.

[0030]FIG. 1 schematically shows the structure of a digital copying machine having a multi-beam exposure device according to an embodiment of the present invention;

[0031]FIG. 2 is a plan view showing an optical path of laser beams in the multi-beam exposure device according to the embodiment, the optical path being developed on the same plane;

[0032]FIG. 3 is a view, including some blocks, for illustrating the structure of a control section in the multi-beam exposure device according to the embodiment;

[0033]FIG. 4 is an exploded perspective view showing the structure of a galvano-mirror in the embodiment;

[0034]FIG. 5 shows the structure of a light-receiving surface of a beam position detection sensor in the embodiment; and

[0035]FIG. 6 is a flow chart illustrating a beam position adjusting process in the embodiment.

DETAILED DESCRIPTION OF THE INVENTION

[0036] An embodiment of the present invention will now be described with reference to the accompanying drawings.

[0037]FIG. 1 schematically shows the structure of an image forming apparatus, that is, a digital copying machine 1, using a multi-beam exposure device as an optical scanning device. The digital copying machine 1 comprises a scanner section 10 and a printer section 20.

[0038] The scanner section 10 includes a first carriage 11, a second carriage 12, an optical lens 13, a photoelectric conversion element 14, an original table 16 and an original hold cover 17.

[0039] The first carriage 11 is formed movable in the direction of the arrow.

[0040] The second carriage 12 follows the movement of the first carriage 11.

[0041] The optical lens 13 imparts predetermined focusing characteristics to light emanating from the second carriage 12.

[0042] The photoelectric conversion element 14, such as a CCD sensor, photoelectrically converts the light provided with the predetermined focusing characteristics by the optical lens 13, thus producing an electric signal.

[0043] The original table 16 serves for placement of the original 15.

[0044] The original hold cover 17 holds the original 15 on the original table 16.

[0045] The first carriage 11 is provided with a light source 18 for illuminating the original 15, and a mirror 11 a for deflecting reflective light from the original 15, which is illuminated with light from the light source 18, toward the second carriage 12.

[0046] The second carriage 12 is provided with a mirror 12 a for deflecting the light from the mirror 11 a of first carriage 11 by 90°, and a mirror 12 b for further deflecting the light from the mirror 12 a by 90°.

[0047] The original 15 placed on the original table 16 is illuminated by the light source 18, and reflects light having a dark/light distribution corresponding to the presence/absence of an image. The reflective light from the original 15 is made incident on the optical lens 13 as image information via the mirrors 11 a, 12 a and 12 b. The incident light on the optical lens 13 from the original 15 is converged through the optical lens 13 on the light receiving surface of the photoelectric conversion element 14. The first carriage 11 and second carriage 12 move along the original table 16 at a relative speed with a ratio of 2:1. Thereby, the image information on the original 15 is read by the photoelectric conversion element 14 and converted to a digital signal representative of the density of the image.

[0048] The printer section 20 comprises a multi-beam exposure device 21 and an image forming section 22 for forming an image on a recording paper sheet P by an electrophotographic method.

[0049] The image forming section 22 includes a photosensitive drum 23, a charger 24, a developing device 25, a transfer device 26, a separator 27 and a cleaning device 28.

[0050] The photosensitive drum 23 is a drum-shaped photosensitive body having a surface formed of a photosensitive layer.

[0051] The charger 24 charges the surface of the photosensitive drum 23 with a potential of a predetermined polarity.

[0052] The developing device 25 applies a toner to, and develops, an electrostatic latent image formed on the surface of the photosensitive drum 23 by the multi-beam exposure device 21.

[0053] The transfer device 26 applies a predetermined electric field to the developed toner image formed by the developing device 25, and transfers the toner image onto the recording paper sheet P.

[0054] The separator 27 separates the recording paper sheet P with the transferred toner image from the photosensitive drum 23.

[0055] The cleaning device 28 removes the untransferred residual toner left on the surface of the photo-sensitive drum 23 after the transfer step, and restores the potential distribution on the photosensitive drum 23 to the pre-charging state.

[0056] The charger 24, developing device 25, transfer device 26, separator 27 and cleaning device 28 are arranged in the named order in the rotational direction of the photosensitive drum 23 (as indicated by the arrow). Laser beams from the multi-beam exposure device 21 are applied to a predetermined position X between the charger 24 and developing device 25.

[0057] In an image processing section (not shown), the image signal corresponding to the original 15 read by the scanner section 10 is subjected to, e.g. contour correction and tone processing for halftone display, and thus converted to a print signal. The print signal is, in turn, converted to a laser modulation signal for modulating the light intensity of laser beams emitted from semiconductor laser elements of the multi-beam exposure device 21.

[0058] The laser beams from the multi-beam exposure device 21 form an electrostatic latent image corresponding to the image signal at a predetermined position X on the photosensitive drum 23. As the photosensitive drum 23 is rotated, the electrostatic latent image is supplied with toner from the developing device 25 and developed. Further, the developed toner image is brought to a position facing the transfer device 26.

[0059] On the other hand, a recording paper sheet P is taken out of a sheet cassette 29 by means of a paper feed roller 30 and a separating roller 31. The recording paper sheet P is aligned and time-adjusted by an aligning roller 32 and conveyed to a position where the transfer device 26 faces the photosensitive drum 23.

[0060] The toner image formed on the photosensitive drum 23 is transferred onto the recording paper sheet P by an electric field applied from the transfer device 26.

[0061] The recording paper sheet P, on which the toner image has been transferred, is separated from the photosensitive drum 23 by the separator 27 and conveyed to a fixing device 34 by means of a convey unit 33. The recording paper sheet P, which has come to the fixing device 34, is heated and pressed so that the toner on the paper sheet P is fixed. The paper sheet P with the fixed toner image is output to a tray 36 by means of output rollers 35.

[0062] On the other hand, after the toner image has been transferred from the photosensitive drum 23 by the transfer device 26, the untransferred residual toner left on the surface of the photosensitive drum 23 is removed by the cleaning device 28. Thereby, the surface potential on the photosensitive drum 23 is restored to the pre-charged initial level in preparation for the next image forming operation.

[0063] The above-described process is repeated to effect successive image formation.

[0064] As is shown in FIG. 2, the multi-beam exposure device 21 includes semiconductor laser elements 41 a and 41 b as light beam generating sources. FIG. 2 shows the multi-beam exposure device 21 in the state in which the optical path of laser beams is developed on the same plane, with the housing (body frame) and some mirrors being omitted.

[0065] The semiconductor laser elements 41 a and 41 b emit laser beams La and Lb with predetermined wavelengths. The laser beams La and Lb pass through pre-deflection optical systems 42 a and 42 b and reach a deflection device 43 having a polygon mirror 43 a. Specifically, the laser beam La passes through the pre-deflection optical system 42 a and then passes through a half-mirror 42 c and a cylinder lens 42 d that effects convergence in a sub-scan direction. Thus, the laser beam La is guided to the deflection device 43 having the polygon mirror 43 a. On the other hand, the laser beam Lb passes through the pre-deflection optical system 42 b and is then reflected by the half-mirror 42 c. The reflected laser beam Lb passes through the cylinder lens 42 d and reaches the deflection device 43 having the polygon mirror 43 a.

[0066] The pre-deflection optical system 42 a adjusts the cross-sectional spot shape of the laser beam La from the semiconductor laser element 41 a into a predetermined shape. The pre-deflection optical system 42 a comprises a finite focus lens 44 a, a stop 45 a and a reflection mirror 46.

[0067] The finite focus lens 44 a effects predetermined convergence of the divergent laser beam from the semiconductor laser element 41 a.

[0068] The stop 45 a adjusts the cross-sectional beam shape of the laser beam, which emanates from the finite focus lens 44 a, into a predetermined shape.

[0069] The reflection mirror 46 deflects the laser beam from the stop 45 a towards the half-mirror 42 c.

[0070] The pre-deflection optical system 42 b adjusts the cross-sectional spot shape of the laser beam Lb from the semiconductor laser element 41 b into a predetermined shape. The pre-deflection optical system 42 b comprises a finite focus lens 44 b, a stop 45 b and a galvano-mirror 47.

[0071] The finite focus lens 44 b effects predetermined convergence of the divergent laser beam from the semiconductor laser element 41 b.

[0072] The stop 45 b adjusts the cross-sectional beam shape of the laser beam, which emanates from the finite focus lens 44 b, into a predetermined shape.

[0073] The galvano-mirror 47 deflects the laser beam from the stop 45 b towards the half-mirror 42 c, and also adjusts the beam positions such that the two laser beams La and Lb have a predetermined inter-beam distance in the sub-scan direction.

[0074] The finite focus lens 44 a, 44 b is, for example, a single lens formed by attaching an ultraviolet-curing plastic aspherical lens to an aspherical glass lens or a spherical glass lens. The galvano-mirror 47 is a mirror with an optical path altering device that can slightly change the direction of reflection of the laser beam to a desired direction.

[0075] The two laser beams La and Lb reflected by the reflection mirror 46 and galvano-mirror 47 are joined via the half-mirror 42 c, and the two joined laser beams L (La+Lb), which have a predetermined inter-beam distance in the sub-scan direction, are guided through the cylinder lens 42 d to the deflection device 43.

[0076] The deflection device 43 is configured such that a polygon mirror having, e.g. eight planar reflection mirrors on its peripheral sides is rotated at a predetermined speed by a motor 43 b.

[0077] A post-deflection optical system 48 is disposed between the deflection device 43 and an image surface (a focal plane in design at a position facing a predetermined position X on photosensitive drum 23). The post-deflection optical system 48 comprises first and second focusing lenses 48 a and 48 b which imparts predetermined optical characteristics to the laser beams L from the deflection device 43.

[0078] A beam position detection sensor 49 and a reflection mirror 50 are also disposed. The beam position detection sensor 49 detects the timing of passing and the position of passing of the laser beams L emanating from the second focusing lens 48 b, before the laser beams L scans an image formation region at the predetermined position X on the photosensitive drum 23. The reflection mirror 50 reflects the laser beams L, which come out of the second focusing lens 48 b, and guides them to the beam position detection sensor 49. The beam position detection sensor 49 is disposed such that the light receiving surface thereof and the surface of the photosensitive drum 23 are positioned at an equal optical distance.

[0079]FIG. 3 is a view, including some blocks, for illustrating the structure of a control section in the multi-beam exposure device 21 according to the embodiment. The control section of the multi-beam exposure device 21 includes a CPU (Central Processing Unit) 51 for controlling the entirety of the device 21. The CPU 51 writes/reads out data in/from a non-volatile memory 52 and an image memory 53.

[0080] The CPU 51 controls a motor driving circuit 54 that controls driving of the motor 43 b of deflection device 43; a beam position detection circuit 55 that receives a detection signal from the beam position detection sensor 49, thereby detecting a beam position; and an operation panel 56.

[0081] The CPU 51 also controls laser drive circuits 57 a and 57 b for driving the semiconductor laser elements 41 a and 41 b; and a galvano-mirror drive circuit 58 having a D/A converter for driving the galvano-mirror 47.

[0082] The semiconductor laser elements 41 a and 41 b are driven by the laser drive circuits 57 a and 57 b and emit laser beams La and Lb at a predetermined timing on the basis of image data stored in the image memory 53.

[0083] In this case, the laser beams are not emitted until the rotation speed of the polygon mirror 43 a of deflection device 43 reaches a predetermined value. When the rotation speed of the polygon mirror 43 a has reached the predetermined value, the CPU 51 outputs predetermined control commands to the laser drive circuits 57 a and 57 b. Upon receiving the predetermined control commands, the laser drive circuits 57 a and 57 b drive the semiconductor laser elements 41 a and 41 b. When laser drive currents have reached predetermined values, the semiconductor laser elements 41 a and 41 b emit laser beams La and Lb.

[0084] As is shown in FIG. 4, the galvano-mirror 47 includes a plate spring 101 for supporting a mirror 100 such that the angle of the mirror 100 is variable, and a frame 102 that supports the plate spring 101. The plate spring 101 is formed of, e.g. beryllium copper, spring stainless steel SU304, etc. The plate spring 101 is provided with a bobbin 103 a and a coil 103 b housed in the bobbin 103 a, which constitute a thrust-producing section 103 that produces a force for altering the position of the plate spring 101.

[0085] The plate spring 101 comprises a support surface 101 a on which the mirror 100 is attached; two hold surfaces 101 b used for attachment to the frame 102; and two torsion bars 101 c that are torsional-deformation portions for coupling each hold surface 101 b and the support surface 101 a. The torsion bars 101 c are vertically twisted so that the plate spring 101 may vertically rotate the mirror 100.

[0086] The plate spring 101, together with spring holders 104 formed of, e.g. resin, is fixed to the frame 102 by means of screws which are driven into screw holes made in the frame 102.

[0087] The coil 103 b houses a magnet 103 c that generates, when current flows in the coil 103 b, a magnetic field for producing a force for vertically rotating the plate spring 101 by a predetermined amount. The magnet 103 c is fixed to a central portion of a fixing plate 105 that is used for fixation to a predetermined location in the exposure device body. The plate spring 101 that supports the mirror 100 is attached to the frame 102, and the frame 102 is integrally attached to the fixing plate 105.

[0088] The bobbin 103 a that incorporates the magnet 103 c and coil 103 b is housed in the frame 102. A damping material such as silicone gel is filled in the gap between the frame 102 and bobbin 103 a so as to prevent vibration of the mirror 100 due to disturbing vibration, etc. The frame 102 functions as a yoke constituting a magnetic circuit for lines of magnetic force from the magnet 103 c.

[0089] The operation of the galvano-mirror 47 will now be described.

[0090] The CPU 51 delivers, e.g. an 8-bit instruction value to the galvano-mirror drive circuit 58. The galvano-mirror drive circuit 58 supplies a current of a predetermined polarity to the coil 103 b of galvano-mirror 47. Thereby, an electromagnetic force is produced between the coil 103 b and magnet 103 c, thereby to twist the support surface 101 a of plate spring 101. Thus, the mirror 100 is vertically rotated by an amount corresponding to the instruction value.

[0091] With the rotation of the mirror 100, the position in the vertical direction, i.e. in the sub-scan direction, of the laser beam reflected from the mirror surface is adjusted. The angle of the mirror 100 is kept by maintaining the current supplied to the coil 103 b. The angle of rotation of the mirror 100 is determined by the instruction value from the CPU 51. In the case of the 8-bit instruction value, the angle of the mirror 100 can be controlled in 256 steps.

[0092] The beam position detection sensor 49, as shown in FIG. 5, includes a right-triangular sensor 49A and three rectangular sensors 49B, 49C and 49D which are arranged equidistantly in parallel so as to face a longer side of two orthogonal sides of the sensor 49A. A distance from a center between the sensors 49B and 49C to a center between the sensors 49C and 49D is 42.3 μm that corresponds to a resolution of 600 dpi.

[0093] The beam position detection circuit 55 receives an output from the beam position detection sensor 49 via a current/voltage converter circuit, an amplifier circuit and an integration circuit, and ultimately binarizes the output and supplies a binary signal to the CPU 51. The output value from the sensor 49A varies from beam position to beam position, since the time of reception of the laser beam varies according to the position of the scanning beam. It is thus possible to detect the position in the sensor 49A where the laser beam has passed, on the basis of the output value from the sensor 49A.

[0094] In the digital copying machine 1, laser beam position adjustment is performed at the time of warm-up after power-on, completion of elimination of jamming, completion of pre-heating, etc., such that an inter-beam distance between a stationary laser beam and a movable laser beam may be set at 42.3 μm corresponding to 600 dpi. Then, the copying operation is started. In this case, the laser beam position adjustment is carried out according to a flow chart of FIG. 6.

[0095] In the state in which a stationary laser beam S is incident on a certain position on the light-receiving surface of the sensor 49A, the CPU 51 effects a control to position a movable laser beam with use of the galvano-mirror 47 within a tolerable range of a beam position A that is a first beam passage target position set between the sensor 49B and sensor 49C. If the movable laser beam is controlled to fall within the tolerable range of beam position A, the CPU 51 turns off the movable laser beam and measures a circuit offset relating to the beam position A (S11).

[0096] Then, the CPU 51 controls the galvano-mirror 47 and exactly sets the movable laser beam at the beam position A by subtracting the measured circuit offset (S12). The CPU 51 reads in an output value DA of the sensor 49A at that time, and stores it in the nonvolatile memory 52 (S13).

[0097] Subsequently, the CPU 51 effects a control to position the movable laser beam with use of the galvano-mirror 47 within a tolerable range of a beam position B that is a second beam passage target position set between the sensor 49C and sensor 49D. If the movable laser beam is controlled to fall within the tolerable range of beam position B, the CPU 51 turns off the movable laser beam and measures a circuit offset relating to the beam position B (S14).

[0098] Then, the CPU 51 controls the galvano-mirror 47 and exactly sets the movable laser beam at the beam position B by subtracting the measured circuit offset (SIS). The CPU 51 reads in an output value DB of the sensor 49A at that time, and stores it in the nonvolatile memory 52 (S16).

[0099] The CPU 51 controls the galvano-mirror 47 such that a difference (DA-DB) between the obtained output values DA and DB of the sensor 49A may become equal to a difference (DS-DM) between the output value DS obtained from the sensor 49A when the stationary laser beam S is incident on the sensor 49A and the output value DM obtained from the sensor 49A when the movable laser beam M is incident on the sensor 49A. Thus, the ultimate beam position of the movable laser beam M is set.

[0100] By the above-described control, the distance between the stationary laser beam S and movable laser beam M is set at 42.3 μm, and the actual exposure is started (start of image forming operation).

[0101] As has been described above, the following process is performed.

[0102] In order to measure a circuit offset at the beam position A, the galvano-mirror 47 is used to effect a control to position the movable laser beam within the tolerable range of the beam position A that is the beam passage target position. Then, the circuit offset is subtracted to exactly set the movable laser beam at the beam position A. The output value DA of the sensor 49A at that time is stored.

[0103] Subsequently, in order to measure a circuit offset at the beam position B, the galvano-mirror 47 is used to effect a control to position the movable laser beam within the tolerable range of the beam position B that is the beam passage target position. Then, the circuit offset is subtracted to exactly set the movable laser beam at the beam position B. The output value DB of the sensor 49A at that time is stored.

[0104] As stated above, the movable laser beam is made to stay in the same position from the time when it is moved to the beam position A until the output value DA is obtained from the sensor 49A. After the output value DA is obtained, the movable laser beam is moved to the beam position B. At the beam position B, the circuit offset is measured and the output value DB is obtained from the sensor 49A.

[0105] As has been described above, according to the embodiment of the invention, the number of control operations for moving the movable laser beam over a long distance with use of the galvano-mirror 47 can be reduced. It is thus possible to decrease the time for position adjustment, which is needed until the movable laser beam M is ultimately set at the position that is distant from the stationary laser beam S by 42.3 μm.

[0106] Furthermore, the laser beam position adjustment can quickly be performed in a short time at the time of warm-up after power-on, completion of elimination of jamming, completion of pre-heating, etc. Thus, the time required until the start of the copying of a first sheet is can shortened.

[0107] In the present embodiment, laser beams are used as light beams. This invention, however, is not limited to this embodiment.

[0108] In this embodiment, the multi-beam exposure device is used as the optical scanning device. This invention, however, is not limited to this embodiment.

[0109] Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. 

What is claimed is:
 1. A beam position adjusting method for an optical scanning device using a galvano-mirror capable of altering an angle of a mirror surface by a drive control, and using a sensor that detects passage positions of a stationary light beam whose scanning position is fixed and a movable light beam whose scanning position is varied by adjusting the angle of the mirror surface of the galvano-mirror, the method comprising: effecting a control to position the movable light beam within the tolerable range of a first passage target position by adjusting the angle of the mirror surface of the galvano-mirror, and measuring a first circuit offset in a detection circuit for an output of the sensor at a time when emission of the movable light beam is stopped; emitting the movable light beam once again, effecting a control to position the movable light beam at the first passage target position with the first circuit offset being subtracted, and storing a first output value from the sensor at a time when the movable light beam is controlled and set at the first passage target position; effecting a control to position the movable light beam within the tolerable range of a second passage target position by adjusting the angle of the mirror surface of the galvano-mirror, and measuring a second circuit offset in the detection circuit for the output of the sensor at a time when emission of the movable light beam is stopped; emitting the movable light beam once again, effecting a control to position the movable light beam at the second passage target position with the second circuit offset being subtracted, and storing a second output value from the sensor at a time when the movable light beam is controlled and set at the second passage target position; and controlling the angle of the mirror surface of the galvano-mirror such that a difference between a third output value from the sensor at a time when the stationary light beam passes through the lightreceiving surface of the sensor and a fourth output value from the sensor at a time when the movable light beam passes through the light-receiving surface of the sensor coincides with a difference between the first output value and the second output value.
 2. The beam position adjusting method for an optical scanning device, according to claim 1, wherein a difference between the first passage target position and the second passage target position is determined according to a resolution.
 3. An image forming method for an image forming apparatus having an optical scanning device using a galvano-mirror capable of altering an angle of a mirror surface by a drive control, and using a sensor that detects passage positions of a stationary light beam whose scanning position is fixed and a movable light beam whose scanning position is varied by adjusting the angle of the mirror surface of the galvano-mirror, the method comprising: effecting, a control to position the movable light beam within a tolerable range of a predetermined first passage target position by adjusting the angle of the mirror surface of the galvano-mirror, and measuring a first circuit offset in a detection circuit for an output of the sensor at a time when emission of the movable light beam is stopped; emitting the movable light beam once again, effecting a control to position the movable light beam at the first passage target position with the first circuit offset being subtracted, and storing a first output value from the sensor at a time when the movable light beam is controlled and set at the first passage target position; effecting a control to position the movable light beam within a tolerable range of a predetermined second passage target position by adjusting the angle of the mirror surface of the galvano-mirror, and measuring a second circuit offset in the detection circuit for the output of the sensor at a time when emission of the movable light beam is stopped; emitting the movable light beam once again, effecting a control to position the movable light beam at the second passage target position with the second circuit offset being subtracted, and storing a second output value from the sensor at a time when the movable light beam is controlled and set at the second passage target position; controlling the angle of the mirror surface of the galvano-mirror such that a difference between a third output value from the sensor at a time when the stationary light beam passes through the light-receiving surface of the sensor and a fourth output value from the sensor at a time when the movable light beam passes through the light-receiving surface of the sensor coincides with a difference between the first output value and the second output value; and controlling an image forming operation when the control for the angle of the mirror surface of the galvano-mirror has been completed.
 4. An image forming apparatus having an optical scanning device using a galvano-mirror capable of altering an angle of a mirror surface by a drive control, and using a sensor that detects passage positions of a stationary light beam whose scanning position is fixed and a movable light beam whose scanning position is varied by adjusting the angle of the mirror surface of the galvano-mirror, the apparatus comprising: first measuring means for effecting, a control to position the movable light beam within a tolerable range of a predetermined first passage target position by adjusting the angle of the mirror surface of the galvano-mirror, and measuring a first circuit offset in a detection circuit for an output of the sensor at a time when emission of the movable light beam is stopped; first memory means for emitting the movable light beam once again, effecting a control to position the movable light beam at the first passage target position with the first circuit offset measured by the first measuring means being subtracted, and storing a first output value from the sensor at a time when the movable light beam is controlled and set at the first passage target position; second measuring means for effecting a control to position the movable light beam within a tolerable range of a predetermined second passage target position by adjusting the angle of the mirror surface of the galvano-mirror, and measuring a second circuit offset in the detection circuit for the output of the sensor at a time when emission of the movable light beam is stopped; second memory means for emitting the movable light beam once again, effecting a control to position the movable light beam at the second passage target position with the second circuit offset measured by the second measuring means being subtracted, and storing a second output value from the sensor at a time when the movable light beam is controlled and set at the second passage target position; first control means for controlling the angle of the mirror surface of the galvano-mirror such that a difference between a third output value from the sensor at a time when the stationary light beam passes through the light-receiving surface of the sensor and a fourth output value from the sensor at a time when the movable light beam passes through the light-receiving surface of the sensor coincides with a difference between the first output value and the second output value; and second control means for controlling an image forming operation when the control for the angle of the mirror surface of the galvano-mirror is completed. 